A serious obstacle to the rational design of innovative approaches for preventing and/or treating osteoporosis is the idiopathic nature of postmenopausal bone loss. Menopause is the most important risk factor for osteoporosis. However, not all postmenopausal women develop osteoporotic fractures indicating that cessation of the menstrual cycle is insufficient to fully account for the disorder. Our working hypothesis is that the denovo production and metabolism of estrogens are among the most important factors influencing the rate of postmenopausal bone loss. Estrone (E1) and its metabolites, 16alpha-Hydroxyl estrone ( (16alpha-OHE1) and 2-hydroxyesterone (2-OHE1), are the most abundant estrogens in postmenopausal women. 16alpha-OHE1 has been recently shown to be a negative risk factor (reduced risk) for postmenopausal bone loss, whereas 2-OHE1 has been positive risk factor (increased risk). 2-OHE1 does not have estrogenic activity in ovariectomized (OVX'd) rats. In contrast, 16alpha-OHE1 appears to be a tissue selective estrogen agonist with a profile of activity similar to the anti-breast drug tamoxifen; 16alpha-OHE1 is a much more effective estrogen agonist on bone and liver than on reproductive tissues. These observations suggest that differences in the skeletal activities on 2-OHE1 and 16-alpha-OHE1 are responsible for the observed association between bone mass and circulating levels of these metabolites in postmenopausal women. We propose to test this hypothesis in ovary intact and OVX'd rats. The specific aims are to determine the dose response effects of 2-OHE1 and 16alpha-OHE1 on the expression of immediate response genes in bone and other estrogen target tissues; and establish the long-term effects of the estrone metabolites on bone architecture, turnover and strength. The proposed research will characterize the probably cellular mechanisms of action. The results of these studies are likely to be relevant to women because of the similarity between postmenopausal bone loss and OVX-induced bone loss in rats, as well as the previous success the rat model has enjoyed for predicting the response of the human skeleton to estrogen agonists and markers to predict the rate of postmenopausal bone loss; 2) manipulation of estrone metabolism by changes in diet or by pharmacological intervention may be a valuable tool for reducing bone loss; and 3) analogs of 16alpha-OHE1 may be useful for prevention and treatment of postmenopausal osteoporosis.